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PHILADELPHIA (CBS) — A testy Doug Pederson revealed that Philadelphia Eagles quarterback Carson Wentz is dealing with a stress injury to his back and is now doubtful for Sunday night’s game against the Los Angeles Rams. The head coach says the injury has “evolved over time.”

“If we put him out there, there has to be 100 percent that there’s no risk of anything further,” Pederson told reporters during a press conference on Friday.

Pederson said that Wentz had a test done on Tuesday that revealed the injury. Wentz does not need surgery and the injury will heal on its own.

Pederson added it’s a type of injury that could take up to three months to fully heal.

Wentz sat out the open portion of practice on Thursday because of his back injury. If Wentz is unable to go, Super Bowl MVP Nick Foles will start for the Eagles at the Rams on Sunday.

Foles hasn’t taken a snap in a game since Week 2. He took over in Week 14 last season after Wentz tore two ligaments in his left knee. Foles led Philadelphia to its first NFL title since 1960 and started the first two games this season, going 1-1.

Wentz has 3,074 yards passing, 21 touchdowns and seven interceptions in 11 games this season after finishing third in the NFL MVP voting last year. He has career highs in passer rating (102.2) and completion rate (69.6 percent).

Foles is 21-12 as a starter, including playoffs, in two stints with Philadelphia.

(© Copyright 2018 CBS Broadcasting Inc. All Rights Reserved. The Associated Press contributed to this report.)